This invention relates to new tetrahydropyrimidines, to a process for their production and to their use as catalysts in the production of polyurethane plastics, including polyurethane foams.
Numerous amines have already been used as catalysts in the production of polyurethanes by reacting polyols with polyisocyanates (see, K. C. Frisch, L. P. Rennao, "Catalysis in Isocyanate Reactions" in J. Macromol. Sci.-Revs. Macromol. Chem. Soc. C 5 (1), 103-150).
In the case of the most active amines which are generally tertiary amines, for example, 1,4-diazabicyclo-(2.2.2)-octane (triethylenediamine, Dabco.RTM.), concentrations of from 0.04 to 0.5 parts by weight, based on the polyol used, are necessary. Other amines have to be used in considerably larger quantities, as is the case where aliphatic isocyanates are used.
However, these small quantities of amine catalyst often involve serious disadvantages. Since the amines used are generally tertiary amines which cannot be incorporated, an unpleasant odor is still in evidence some time after production. This applies in particular to articles of everyday use, such as upholstery, motor vehicle fittings, shoes and furniture. Also, these amines are frequently responsible for the yellowing of light-colored leather or plastics surfaces.
Another disadvantage of non-incorporable amine catalysts of the type in question is that they often have a tendency to exude to the surface by other physical and/or chemical processes. Thus, a white coating in which diazabicycloundecene is found can be formed, for example, on a diazabicycloundecene-catalyzed polyurethane material of the type used in the production of steering wheels and headrests. Similarly, where triethylene diamine is used as catalyst in the production of polyurethane shoe soles, discoloration is frequently observed in light-colored upper leathers, making it impossible for the sole to be directly foamed onto the shoe upper. This discoloration is produced by triethylene diamine escaping during the foaming reaction. A similar effect is observed where non-incorporable tetrahydropyrimidines are used as catalyst.
Acyclic amidines as catalysts in the production of polyurethanes from aliphatic isocyanates are described in German Offenlegungsschrift No. 1,950,262. German Offenlegungsschrift No. 2,737,671 describes the use of combinations of cyclic amidines, which have tetrahydropyrimidine or imidazoline structures, and metal salts as catalysts for the production of polyurethanes.
Published Japanese Patent Application No. 7,102,672 describes the use of tetrahydropyrimidines for catalyzing urethanization reactions. Bicyclic amidines as polyurethane catalysts are described in German Offenlegungsschrift No. 1,745,418.
However, these catalysts still do not meet the practical needs for high catalytic activity and odorlessness of the plastics material obtained.
German Offenlegungsschrift No. 2,601,082 already describes the production of polyurethanes using incorporable catalysts of the amidine type. However, the aminopyridines mentioned in that reference show only sufficient catalytic activity for the production of polyurethanes from aromatic polyisocyanates, being too weak for the production of polyurethanes from aliphatic polyisocyanates. Accordingly, the scope of application of aminopyridines as catalysts in the production of polyurethanes is often too narrow for practical purposes.
Now, it is surprisingly possible by virtue of the present invention to overcome the disadvantages shown from the prior art. The tetrahydropyrimidines disclosed and used in accordance with the invention are excellent catalysts for the production of polyurethanes both from aliphatic and also from aromatic polyisocyanates.
Also, the polyurethane plastics produced with them show no noticeable amine odor.
The tetrahydropyrimidines according to the invention have a very much lower vapor pressure than the cyclic amidines according to German Offenlegungsschrift No. 1,950,262, the bicyclic amidines according to German Offenlegungsschrift No. 2,737,671 and the tetrahydropyrimidines according to Japanese Patent Application No. 7,102,672.
In addition, as catalysts:
1. In general, they are highly compatible with the PU-matrix;
2. In many cases, they have a particularly high content of catalytically active groups in the molecule; and
3. They are more stable to hydrolysis than the hitherto known monocyclic tetrahydropyrimidines.
There are already numerous other processes for the production of 1-alkyl-2-methyl-tetrahydropyrimidines, including, for example, the reaction of N-alkyl propylene diamines with open-chain imido acid esters or amidines (A. Pinner, Die Chemie der Imidoether und ihrer Derivate (The Chemistry of the Imido Ethers and Their Derivatives), R. Oppenheim, Berlin, 1892); the reaction of N-alkyl propylene diaminotoluene sulfonic acid salts with carboxylic acid nitriles (J. Chem. Soc. 1947, 497); the hydrogenation of N-acylaminonitriles with the N-acyl-N-alkylpropylene diamines formed being dehydrated under the reaction conditions to form 1-alkyl-2-methyl tetrahydropyrimidines (J. Am. Chem. Soc. 71, 2350 (1949)); and the reaction of N-alkyl propylene diamines with oxazolines (DE-OS No. 2,154,948). However, many of these processes have the disadvantage that the reaction is not complete and, in particular, large quantities of unwanted secondary products are formed. The reaction temperature in all these processes is in the range of from 100.degree. to 200.degree. C., the reaction being carried out in the presence of catalysts, for example, acid compounds (e.g., hydrochloric acid or toluene sulfonic acid) or metal compounds (e.g., Ni, Co or Cu).
The reaction, as described in Chem. Ber. 98, 3652 (1965), of N-ethyl propylene diamine with acetoacetic acid ethyl ester in the presence of toluene sulfonic acid at high reaction temperatures of 210.degree. C. is also known. This process is described with reference to a 0.2 molar reaction batch which unfortunately cannot be scaled up to industrial levels.
As cyclic amidines, tetrahydropyrimidines are extremely sensitive to hydrolysis. At the temperature mentioned and in the presence of acid compounds such as toluene sulfonic acid, the cyclic amidine is immediately hydrolyzed by the water formed during the condensation reaction. The N-acyl-N-ethyl propylene diamine formed by hydrolysis may be slowly recycled with elimination of water under the conditions prevailing during working up by distillation, and this may be done in an entirely acceptable time in the case of a 0.2 molar batch but, where this process is carried out commercially with batches of 100 kg and larger, this leads to a completely unsatisfactory volume-time yield. Working up of the ternary mixture which accumulates, consisting of tetrahydropyrimidine, its hydrolysis product N-acyl-N-alkyl propylene diamine and water is extremely complicated and requires a time-consuming and complex distillation process. In addition, a very high percentage of unusable residues is obtained in consequence of the thermal stressing of the reaction mixture, significantly reducing the yield of pure product.
In addition, the production of N-substituted tetrahydropyrimidines from N-alkyl propane diamines and .DELTA..sup.2 -oxazolines in commercially inadequate yields of 19 to 73% is described in Synthesis 1972, No. 1, page 37.
Accordingly, another object of the present invention is to develop a process which enables tetrahydropyrimidines corresponding to general formulae disclosed below to be obtained in high yields and in high degrees of purity.